The invention relates to a directly heated thermionic flat emitter whose emitting surface has conductor tracks which are formed by slots in the emitting surface and are supported by emitter legs which form current conductors, the conductor tracks being formed by a grid-like pattern of cuts composed of a multiplicity of straight slots which run at least transverse to the course of the main current direction and are arranged in a plurality of rows offset from one another.
Thermionic flat emitters such as are described, for example, in DE 100 16 125 A1, which is not a prior publication, are used in X-ray tubes, in particular in rotary piston X-ray tubes. The part of the emitter forming the emitting surface is generally of disk-shaped design and comprises a thin, slotted tungsten sheet with a thickness of approximately 100 to 200 xcexcm. The current is conducted via emitter legs which are situated diametrically opposite one another and adjoin the emitting surface.
In order to achieve electron emission in the X-ray tube, the emitting surface has to be heated up to above 2000xc2x0 C. At these high temperatures, the stability of tungsten is already diminished as safe. In addition, xe2x80x9ccreeping effectsxe2x80x9d which have a negative effect on the shaping of the emitter occur owing to the high centrifugal forces, which are produced by the selfrotation of the rotary piston X-ray tube, and also by mounting stresses. This influence also particularly has a disadvantages effect on the shape of the focal spot or on the configuration of the focal spot. Geometrical changes in the emitter during the lifetime of the tube therefore have to be recallibrated by altered external magnetic fields to the extent this is at all possible.
In the design of a thermionic flat emitter for use in a rotary piston tube, it is therefore necessary to take into account that tungsten loses strength at high temperatures, in particular above 2000xc2x0 C., and to take care that temperature changes as small as possible occur over the mostly circular emitting surface, and that there is as high a mechanical strength as possible in order to keep the mechanical distortion of the emitter as low as possible given the high temperatures and the loads from the centrifugal forces.
DE 27 27 907 A1 discloses a directly heated thermionic flat emitter of the type mentioned at the beginning, whose emitting surface has meandering conductor tracks which run transverse to the main current direction and are formed by slots in the emitting surface and are supported by emitter legs which form current conductors, the conductor tracks being arranged in two rows and forming a single current path.
Moreover, DE-C 336 781 discloses a directly heated thermionic emitter whose emitting surface is supported by emitter legs, which form current conductors, and has conductor tracks which run transverse to the main current direction and are formed by slots in the emitting surface.
DE-C 486 245 discloses a luminous element for electric incandescent lamps, which is designed as a hollow body which is essentially of rotationally symmetric design and is provided with openings which allow blackbody radiation to emerge.
Although the emitter described in the patent application mentioned at the beginning offers good conduction of current with its spirally wound conductor tracks, and although this emitter design already achieves a longer service life than emitters of coiled design, the aim is to achieve further improvement under the above named points of view.
The invention specified below is therefore based on the object of improving an emitter of the generic type mentioned at the beginning to the effect that a higher mechanical stability, in particular a higher strength against distortion, is achieved, and that no destruction of the emitter can occur owing to ions being welded on, which would result in immediate failure of the X-ray tube.
It may be remarked with reference to the last named effect that during operation of an X-ray tube the interaction of the focused electrons with the tungsten anode produces a strong local temperature rise which can lead to fusing of the tungsten on the anode. The temperature rise at the anode leads, in turn, to gases being released from the anode and its surroundings which lead to a pressure rise in the tube. Moreover, tungsten vapour is also produced by the fusing process. If molecules of gas or vapour are now located in the region of high field strengths between the cathode and anode, said molecules are partially ionized and accelerated in the direction of the cathode. The course of the field lines gives rise to a focusing effect, which means that the ion beam impinges in a concentrated fashion on the emitter and has a very strong local heating effect on the tungsten sheet there. The tube current also rises sharply owing to this additional energy input. Local, partial instances of fusing with attendant xe2x80x9cburning outxe2x80x9d of the emitter can result in the case of a lengthy operating time, and thus a lengthy exposure time.
In the emitter according to the invention, the conductor tracks are formed by a grid-like pattern of cuts which comprises a multiplicity of straight slots which run at least transverse to the main current direction and are arranged in a plurality of rows offset from one another, a multiplicity of meandering current paths being formed thereby.
The particular advantages of such a design are that the strength of the current tracks against distortion is improved, that a virtually isotropic state with uniform current density in all directions is achieved, and that no destruction of the emitter can occur owing to ions being welded on. Moreover, such a design permits a closed outer contour, resulting in a further increase in mechanical stability. The design of the conductor tracks according to the invention finally also permits easier monitoring of the state of the emitter without demounting the tube, for example by identifying the state of the emitter by resistance measurement or detecting the welding-on of ions or by determining changes in focal spot, it thereby being possible to determine the residual lifetime of the tube.
In accordance with an advantageous design, the pattern of cuts forms a multiplicity of parallel, meandering current paths. Owing to the multiplicity of meandering turns, the cross-section and the length of the individual current tracks, and thus the electrical resistance of the individual meanders can be set such that the required current intensity is not higher overall, and can, rather, be kept lower than in the case of spirally running conductor tracks of known designs.
It is particularly advantageous when the individual cells of the rectangular structure of cuts have additional diagonal cuts. It is possible thereby to achieve an even more homogenous current distribution. As a rule, it is possible for the current to be conducted as usual by two emitter legs; the stability can be increased, however, when, as proposed in accordance with a further advantageous variant, four emitter legs are provided.